Antenna arrays have found numerous applications in wireless communication systems, where a first communication device comprising a plurality of antennas transmits signals to a second communication device that comprises at least one or a plurality of antennas. For example, the first communication device may be a base station and the second communication device may be one of a plurality of mobile stations served by the base station. When the first communication device transmits to the second communication device, the first communication may beamform the transmission via its plurality of antennas to the second communication device, and likewise if the second communication device has multiple antennas, it may beamform its transmissions to the first communication device. The term “beamform” as used herein means that the signals received by a communication device on its multiple antennas from another device are coherently combined, and the signals transmitted from multiple antennas of a communication device are weighted in both phase and magnitude so that they will be coherently combined at the intended destination communication device.
Beamforming a transmission from one communication device to another communication device works well because the beamforming weights used for beamforming a transmission are selected or computed for an intended destination communication device so that the beam is directionally focused to the intended destination device. This is the case for so-called “unicast” messages that are intended for a particular destination device.
There is also a need to transmit “broadcast” or “multicast” messages that are intended for reception by multiple communication devices. However, broadcast messages cannot be beamformed in the convention sense because they are intended for reception by many or all mobile stations in a coverage region or cell. If a base station uses a radiation beam pattern that covers the entire cell to transmit a broadcast message, the broadcasted signal may scatter over entire cell and may not be reliable received by any mobile station in the coverage region. There will also be no beamforming gain achieved for the broadcast message that is sent by a radiation beam to the entire cell.
There are still other situations when a broadcast message is transmitted even though the message is actually intended for a specific mobile station. For example, when the location or spatial signature of the specific mobile station is not known, the only option available to the base station is to transmit a broadcast “paging” message in order to locate that mobile station within the coverage region.
Beamforming techniques heretofore known are not useful when transmitting a broadcast message that is intended for reception by multiple mobile stations within a coverage region of a base station. Even if a broadcast message is transmitted with a radiation beam pattern for the entire coverage region, the signal level associated with the broadcast message will be lower than that for a unicast beamformed message and the coverage or range of the broadcast message will be limited.